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Dear Yaroslav,</div>
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In the figure 3 of your paper, the nucleon acts as a spectator in the left two diagrams (SM part). Therefore, the invariant mass of final state particles (such as pi0 and 2 gammas) should be the mass of the initial decaying meson. The right two diagrams (V_B
part) involves interactions with nucleon, as a result, the reconstructed invariant mass of final state particles (pi0, 2 gammas) may not be the same as the initial meson.</div>
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For all experimental analysis (including A2 and AGS papers), one alyway requires that the invariant mass of final state particles (pi0, 2 gammas) matches the mass of the initial decaying meson. The V_B may contribute to the experimental result due to non-perfect
experimental resolutions. Therefore, the V_B comes in the game only partially. In other word, the new physics contribution may depend on the experimental sensitivities.</div>
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Best,</div>
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Liping</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Alexander Somov <somov@jlab.org><br>
<b>Sent:</b> Tuesday, November 18, 2025 5:11 PM<br>
<b>To:</b> Yaroslav Balytskyi <hr6998@wayne.edu>; Gan, Liping <ganl@uncw.edu>; Dave Mack <mack@jlab.org><br>
<b>Subject:</b> Re: Photoproduction test of the solution to the KLOE–MAMI η→π⁰γγ discrepancy.</font>
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<p style="color:#007073"><b>[<i>This email originated from outside of UNCW</i>]</b></p>
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Dear Yaroslav,</div>
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Thanks a lot for the information. Interesting, I'll take a look.</div>
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Cheers,</div>
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Sasha</div>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Yaroslav Balytskyi <hr6998@wayne.edu><br>
<b>Sent:</b> Tuesday, November 18, 2025 4:08 PM<br>
<b>To:</b> ganl@uncw.edu <ganl@uncw.edu>; Alexander Somov <somov@jlab.org>; Dave Mack <mack@jlab.org><br>
<b>Subject:</b> [EXTERNAL] Photoproduction test of the solution to the KLOE–MAMI η→π⁰γγ discrepancy.</font>
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Dear All!</div>
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As you probably know, the recent KLOE measurement of the η →π⁰γγ decay: <a data-auth="NotApplicable" data-linkindex="0" rel="noopener noreferrer" title="https://arxiv.org/abs/2505.09285" class="x_x_x_OWAAutoLink" id="OWA9ef97e51-3834-8830-f86a-439c99062c50" target="_blank" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2505.09285&d=DwMGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UEJkvUcBvE4mBa6l1pQs5Q&m=y2yvftm61UrsjzrRwHxHz2jA59FM1oc6q_jEfPjqFG8je0Yi5KeuhmPytHwI52p1&s=0n0_ahtSxYfU_jAyg4MvEF469KMUgpAOmyzdw9AYjfc&e=" originalsrc="https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2505.09285&d=DwMGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UEJkvUcBvE4mBa6l1pQs5Q&m=y2yvftm61UrsjzrRwHxHz2jA59FM1oc6q_jEfPjqFG8je0Yi5KeuhmPytHwI52p1&s=0n0_ahtSxYfU_jAyg4MvEF469KMUgpAOmyzdw9AYjfc&e=" style="margin:0px">https://arxiv.org/abs/2505.09285</a>,
finds the result twice smaller than the current world average. </div>
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The PDG - KLOE difference is around 5.5 σ. </div>
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I propose a solution to this discrepancy via a new nucleon-triggered vector VB: <a data-auth="NotApplicable" data-linkindex="1" rel="noopener noreferrer" title="https://arxiv.org/abs/2507.09436" class="x_x_x_OWAAutoLink" id="OWA3b695aef-bb2f-8f64-22cb-2530f09d2c20" target="_blank" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2507.09436&d=DwMGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UEJkvUcBvE4mBa6l1pQs5Q&m=y2yvftm61UrsjzrRwHxHz2jA59FM1oc6q_jEfPjqFG8je0Yi5KeuhmPytHwI52p1&s=w1upi2cN0KhyD0LDVquxGs2jr48WXLRlI0pI15ohEr0&e=" originalsrc="https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2507.09436&d=DwMGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UEJkvUcBvE4mBa6l1pQs5Q&m=y2yvftm61UrsjzrRwHxHz2jA59FM1oc6q_jEfPjqFG8je0Yi5KeuhmPytHwI52p1&s=w1upi2cN0KhyD0LDVquxGs2jr48WXLRlI0pI15ohEr0&e=" style="margin:0px">https://arxiv.org/abs/2507.09436</a></div>
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<span style="color:rgb(0,0,0)"> </span><span style="color:rgb(200,38,19)"><b>If </b></span><span style="color:rgb(0,0,0)">the KLOE-MAMI discrepancy is real (not experimental systematics), the same mechanism will inevitably manifest itself in the 2 -> 3 photoproduction
via: </span></div>
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γ + p → VB + π/η/η' + p</div>
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There can be three options: </div>
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1) VB invisible (search via missing mass)</div>
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2) VB decays to an invisible dark pion and the visible photon (tagging on the extra photon)</div>
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3) VB decays to visible dark pion and visible photon (visible ALP particle, visible SM pseudoscalar, visible photon). </div>
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As far as I understand, GlueX already measures 2 -> 3 reactions γp → ω π0p and γp →ω ηp, and therefore should be in an excellent position to confirm or exclude this scenario with a dedicated analysis.</div>
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Please see the arXiv files I'm referring to and the illustration "Photoproduction_VB_nucl.pdf" attached. </div>
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If you are interested, I can provide more details. </div>
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BR, YB</div>
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